Mesoporous FCC Catalyst Formulated With Gibbsite

This invention relates to mesoporous catalytic cracking catalysts. More particularly, catalytic cracking catalysts formulated with gibbsite show improved coke selectivity and greater catalyst strength.

Fluid catalytic cracking (FCC) is a well-known method for converting high boiling hydrocarbon feedstocks to lower boiling, more valuable products. In the FCC process, the high boiling feedstock is contacted with a fluidized bed of catalyst particles in the substantial absence of hydrogen at elevated temperatures. The cracking reaction typically occurs in the riser portion of the catalytic cracking reactor. Cracked products are separated from catalyst by means of cyclones and coked catalyst particles are steam-stripped and sent to a regenerator where coke is burned off the catalyst. The regenerated catalyst is then recycled to contact more high boiling feed at the beginning of the riser.

Typical FCC catalysts contain active crystalline aluminosilicates such as zeolites and active inorganic oxide components such as clays of the kaolin type dispersed within an inorganic metal oxide matrix formed from amorphous gels or sols which bind the components together on drying. It is desirable that the matrix be active, attrition-resistant, selective with regard to the production of hydrocarbons without excessive coke make and not readily deactivated by metals. Current FCC catalysts may contain in excess of 40 wt. % zeolites. At these high zeolite concentrations, it is difficult to maintain a pore structure that is highly mesoporous while at the same time highly active and selective.

U.S. Pat. No. 5,221,648 describes a FCC catalyst which is a composite of a crystalline aluminosilicate zeolite within a mesoporous silica-alumina matrix. The matrix has a polymodal pore size distribution and is attrition-resistant and selective in the production of olefins. U.S. Pat. No. 4,908,405 relates to a FCC process employing a catalyst composition comprised of a monodispersed mesoporous aluminosilicate matrix material having pore diameter between about 100 and 500 Angstroms, alumina and a crystalline zeolite. U.S. Pat. No. 4,010,116 is directed to zeolite catalysts having improved thermal stability. The catalysts incorporate a synthetic mica-montmorillonite aluminosilicate, in admixture with a pseudoboehmite. The pseudoboehmite may contain aluminum trihydroxide components such as bayerite and gibbsite. Upon calcination at 500.degree. C., pseudoboehmite converts to gamma alumina. Therefore, a fresh catalyst containing pseudoboehmite would contain increasing amounts of gamma alumina as it ages in the FCC unit.

It would be desirable to use gibbsite in the inorganic matrix of a FCC catalyst because it is abundant and inexpensive. However, gibbsite is known to have a low surface area and is relatively inert in terms of its activity and has therefore been little used in FCC catalysts.

It has been discovered that a mesoporous FCC catalyst having improved coke selectivity and catalyst strength can be formulated using gibbsite. Accordingly the present invention relates to a catalytic cracking catalyst having improved selectivity for conversion over coke formation which comprises:

(a) from 1 to 80 wt. %, based on catalyst composition, of a crystalline aluminosilicate zeolite having a silica to alumina mole ratio of about 2.0 or greater;

(b) from 5 to 50 wt. %, based on catalyst composition, of gibbsite; and

(c) a silica matrix, wherein the silica matrix is prepared from at least one of (i) a silica sol made by an ion-exchanged process, and/or (ii) an acidic silica sol prepared by mixing sodium silicate, an acid and an aluminum salt, and an aluminum salt of a mineral acid provided that the sol is not converted to a gel during catalyst preparation.

Another embodiment of the invention relates to a fluid catalytic cracking process for converting a hydrocarbon feedstock into lower boiling products which comprises contacting the hydrocarbon feedstock with a fluidized bed of the catalytic cracking catalyst having improved selectivity for conversion over coke formation described above under catalytic cracking conditions.

FIG. 1a shows the porosity of both pseudoboehmite and gibbsite-containing catalysts after calcining.

FIG. 1b shows the porosity of pseudoboehmite and gibbsite-containing catalsyts after calcining and steaming the catalysts at 1 atm. and 760.degree. C.

FIG. 2 demonstrates that pure gibbsite does not develop mesoporosity in the 100-500 Angstrom range while gibbsite incorporated into a FCC catalyst with a silica does develop mesoporosity.

FIG. 3 demonstrates that a pseudoboehmite does not develop mesoporosity either alone or in a catalyst formulation after heating to 760.degree. C.

FIG. 4 is a comparison of specific coke for a gamma alumina (steamed pseudboehmite)-containing catalyst vs. a steamed gibbsite-containing catalyst.

The FCC catalyst according to the invention is comprised of a matrix material having dispersed therein a crystalline aluminosilicate zeolite and gibbsite.

The matrix material is mesoporous having pore diameters in the range between about 100 to 300 .ANG. and may contain two different types of silica sols, and preferably a clay component. A first type of silica sol is made by an ion-exchange process. Monodispersed silica sols are commercially available under trade names such as LUDOX (E.I. DuPont), NALCOLAG (Nalco Chemical Co.) and NYACOL (PQ Corporation). The second type of silica sol is prepared by reacting sodium silicate with a mineral acid followed by addition of an aluminum salt. The type of mineral acid is not critical, sulfuric acid is preferred. The aluminum salt is preferably the salt of a mineral acid, e.g., hydrated aluminum sulfate.

Virtually any crystalline aluminosilicate zeolite having catalytic cracking activity can be used in the practice of this invention. Such zeolite materials are described in U.S. Pat. Nos. 3,660,274 and 3,944,482, both of which are incorporated herein by reference. Non-limiting examples of such zeolites which can be employed in the practice of this invention, include both natural and synthetic zeolites. These zeolites include zeolites of the structural types included in the "Atlas of Zeolite Structural Types" edited by W. M. Meier and D. H. Olson and published by the Structure Commission of the International Zeolite Association in 1987 and also included herein by reference. Preferred are the faujasites, more preferred are zeolites X and Y, which are faujasite-type zeolites, and most preferred is zeolite Y. The term faujasite-type, as used therein, means those zeolites isostructural to faujasite.

Zeolites typically have silica-to-alumina mole ratios SiO.sub.2 /Al.sub.2 O.sub.3 of at least about 2 and uniform pore diameters from about 3 to 15 Angstroms. Zeolites as produced or found in nature normally contain an alkali metal cation, such as sodium and/or potassium and/or an alkaline earth metal cation, such as magnesium and/or calcium. When used as a hydrocarbon cracking catalyst component, it is usually necessary to decrease the alkali metal content of the crystalline zeolite to less than about 5 wt. %, preferably less than about 1 wt. %, and more preferably less than about 0.5 wt. %. The alkali metal content reduction, as is known in the art, may be conducted by exchange with one or more cations selected from the Groups IIB through VIII of the Periodic Table of Elements (the Periodic Table of Elements referred to herein is given in Handbook of Chemistry and Physics, published by the Chemical Rubber Publishing Company, Cleveland, Ohio, 45th Edition, 1664), as well as with hydronium ions or basic adducts of hydronium ions, e.g., NH.sub.4 +, capable of conversion to a hydrogen cation upon calcination. Preferred cations include rare earths, calcium, magnesium, hydrogen and mixtures thereof. Ion-exchange methods are well known in the art and are described, for example, in U.S. Pat. No. 3,140,249; U.S. Pat. No. 3,142,251 and U.S. Pat. No. 1,423,353, the teachings of which are hereby incorporated by reference. The concentration of the hydrogen cation in the finished catalyst is the difference between the theoretical exchange capacity of the particular zeolite and the number of equivalents of rare earths, alkaline earths, and alkali metals actually present. The particle size of the zeolite component may range from about 0.1 to 10 microns, preferably from about 0.5 to 3 microns. The amounts of zeolite component in the total catalyst will generally range from about 1 percent to about 80 wt. percent, preferably from about 5 percent to about 70 wt. percent, more preferably from about 10 percent to about 60 wt. percent based on the total weight of catalyst. The silica:alumina weight ratio of the catalyst ranges from about 4:1 to about 1:4, preferably from about 1:2 to about 2:1.

Gibbsite has the chemical formula Al(OH).sub.3 and is the principal constituent of bauxites. Gibbsite has a monoclinic crystal symmetry with four molecules per unit cell. It is commercially available from Alcoa in particles sizes ranging from 0.3 to 2.0 microns under the tradename SPACERITE. The amount of gibbsite is preferable from about 10 to about 40 wt. %, based on catalyst composition.

In a first step of the catalyst preparation procedure, zeolite, gibbsite and preferably one or more clays and zeolite are added together or in sequence, in any order, and slurried at ambient temperature in a limited, controlled, and critical amount of water. In general, it has been found that the weight ratio of water:solids in the slurry can range between about 1.5:1 to about 4:1, preferably between about 1.7:1 to about 2:1. A weight ratio of water:solids approximating about 1.7 has been found highly successful in forming high quality catalysts. When the weight ratio of water:solids is less than about 1.4, the viscosity of the slurry is too high to spray dry, and when the weight ratio of water:solids exceeds about 4:1 the attrition-resistance of the catalyst is poor. The clay component added to, or slurried with the zeolite is typically a naturally occurring, relatively non-porous clay such as kaolin, bentonite, hectorite, sepiolite, attapulgite, montmorillonite, halloysite or the like. The pH of the slurry at this time ranges between about 4 and about 10; and the addition of the clay to the slurry does not significantly alter the pH of the slurry, if at all.

In a second step, a silica sol obtained or derived from an ion-exchange process is added to the previously prepared zeolite, or zeolite/clay(s) slurry. The silica sol is one of controlled particle size; the particles of silica ranging from about 4.0 nm (nanometers) to about 22.0 nm, preferably from about 10.0 nm to about 15.0 nm average diameter. (The Chemistry of Silica: Solubility, Polymerization, Colloid and Surface Properties, and Biochemistry by Ralph K. Iler, Wiley, N.Y., 1979.) Thus, a monodispersed colloidal silica, or silica of controlled particle size, derived from an ion-exchange process, e.g. LUDOX, is added to the zeolite/clay(s) slurry, if necessary with enough additional water to maintain the water:solids weight ratio between about 1.5:1 and 4:1, preferably about 1.7:1 to 2:1, and the mixture thoroughly stirred to abrade and eliminate crumbs. Suitably, the slurry is admixed and abraded on a colloid mill to produce particles ranging in size from about 1-3 microns. The pH of the slurry at this time is about neutral, or ranges generally between about 4 and 10.

Silica sols suitable for use in the present invention are any of those derived from an ion-exchange process which have a substantially uniform particle size within the range of about 20 to 400 Angstroms. The term, substantially uniform, as used herein with respect to the particle size means that at least 80%, preferably at least 90%, of the particles have a particle size from about 0.5D to 1.5D, where D is the median particle diameter. It is preferred that the silica sols used herein have spheroid particle shapes. These silica sols can be prepared by any conventional method in the art and examples can be found in The Chemistry of Silica: Solubility, Polymerization, Colloid and Surface Properties, and Biochemistry by Ralph K. Iler, Wiley, N.Y., 1979.

An aluminum salt, e.g. aluminum sulfate, can be added to the admixture subsequently to the silica sol derived from the ion-exchange process.

In the next, or third step, a second type of silica sol prepared by the reaction of a sodium silicate solution and a solution of a mineral acid and the salt of a mineral acid, as e.g. a solution of sulfuric acid and aluminum sulfate, with an aluminum salt, unless the aluminum salt has been previously added, is blended with the zeolite/clay(s)/silica sol slurry, additionally adding water if necessary to maintain the water:solids ratio between about 1.5:1 to 4:1, preferably about 1.7:1 to 2:1. A sufficient amount of the acid silica sol is added to adjust the pH within a range of from about 2 to 5, preferably from about 2.5 to about 4.5. The density of the slurry, on completing the addition of the acid silica sol, preferably ranges from about 1.16 to about 1.25, and more preferably from about 1.18 to about 1.22.

After blending the acid silica sol, and aluminum salt, with the zeolite/clay(s)/silica sol slurry, with adjustment of the water content, density, and preferably also the viscosity, the slurry, now containing all essential components, is now ready for feeding to a spray drier to form spherical particles.

In a fourth step, the slurry suitably at or below ambient temperature is fed to a drier, preferably a spray drier, at a temperature sufficient to remove the water and form micro spheres of average particle diameter ranging from about 10 microns to about 200 microns, preferably from about 60 microns to about 100 microns. The temperature is sufficiently high to dry the slurry and form a rigid structure, but insufficiently high as to cause alkali metal components to be occluded within the zeolite and prevent it from being washed, ion-exchanged, and removed from the zeolite. Typically, the slurry is fed to a drier, preferably a spray drier at an average inlet temperature ranging from about 250.degree. C. to about 350.degree. C., and an outlet temperature ranging from about 125.degree. C. to about 225.degree. C. Surprisingly, the mesoporosity of the matrix material, and mechanical strength of the catalyst, is dependent on the amount of water contained in the slurry. It is essential that the water: solids content of the slurry range between about 1.5:1 to about 4:1, preferably from about 1.7:1 to about 2:1, and the density of the slurry be greater than 1.16 g/cc, and preferably greater than 1.18 g/cc to form catalysts of this invention.

In a fifth step, the particles of microspherical shape are washed with deionized water at temperatures such as about 50.degree. C., and ion-exchanged by contact with an ammonium salt solution of a strong acid for a time sufficient to remove the alkali metal, e.g. sodium, from the zeolite. Exemplary of ammonium salts which can be used in solution for this purpose are ammonium sulfate, ammonium chloride, ammonium nitrate and the like. The ion-exchanged particles are generally again washed, e.g. at about 50.degree. C. Typically, the zeolite portion of the catalyst, after ion-exchange, and washing, contains less than about 1.0 percent alkali metal, and generally less than about 0.5 percent of the alkali metal. The non-zeolitic portion of the pre-catalyst, or matrix, prior to steaming, contains pores of bimodal distribution; a distribution which after steaming is shifted to pore distributions, as measured by mercury porosimetry, wherein at least 75 percent, and preferably from about 80 percent to about 90 percent, of the pore volume between 45 Angstroms and 2000 Angstroms is in pores of diameter greater than 160 Angstroms. Generally, and preferably, after steaming, the pore distribution of the matrix material is polymodal wherein, of the pore volume measured between 45 Angstroms and 2000 Angstroms, a first mode of which is characterized as having at least about 75 percent, and preferably from about 80 percent to about 90 percent, pore sizes greater than 160 Angstroms in diameter, and a second mode wherein up to 20 percent, and preferably from about 5 percent to about 10 percent, of the pore sizes are greater than 100 Angstroms in diameter, but less than 160 Angstroms in diameter. In its most preferred aspects the matrix material, after steaming, is bimodal and is characterized as having a first mode wherein at least about 75 percent, preferably from about 80 percent to about 90 percent, of the pore volume between 45 Angstroms and 2000 Angstroms is in pores greater than 160 Angstroms diameter, and more particularly the pores are of diameter ranging between about 160 Angstroms and 400 Angstroms diameter, more particularly between about 200 Angstroms and 250 Angstroms diameter, and a second mode wherein up to than about 20 percent, and preferably up to about 10 percent of the pores are of diameter exceeding 100 Angstroms diameter. Generally, and preferably at least about 5 percent, more preferably from about 5 percent to about 10 percent, of the pores are of a size distribution ranging between about 100 Angstroms and 160 Angstroms, preferably between about 110 Angstroms and 150 Angstroms. These measurements are taken after steaming the particles at 760.degree. C. in 1 atmosphere of steam for a period of sixteen hours.

In conducting the catalytic cracking operation, utilizing the catalyst of this invention, the temperature employed ranges from about 750.degree. F. (399.degree. C.) to about 1300.degree. F., (704.degree. C.) preferably from about 900.degree. F. (482.degree. C.) to about 1050.degree. F., (566.degree. C.) and the pressure employed is one ranging from about 0 psig (101 kPa) to about 150 psig (1136 kPa), preferably from about 0 psig (101 kPa) to about 45 psig. (411 kpa). Suitably, catalyst/oil ratios in the cracking zone used to convert the feed to lower boiling products are not more than about 30:1, and may range from about 20:1 to about 2:1, preferably from about 4:1 to about 9:1. The catalytic cracking process may be carried out in a fixed bed, moving bed, ebullated bed, slurry, transfer line (dispersed phase) or fluidized bed operation. Suitable regeneration temperatures include a temperature ranging from about 1100 to about 1500.degree. F. (593 to about 816.degree. C.), and a pressure ranging from about 0 to about 150 psig (101 to about 1136 kpa). The oxidizing agent used to contact the partially deactivated (i.e., coked) catalyst will generally be an oxygen-containing gas such as air, oxygen and mixtures thereof The partially deactivated (coked) catalyst is contacted with the oxidizing agent for a time sufficient to remove, by combustion, at least a portion of the carbonaceous deposit and thereby regenerate the catalyst in a conventional manner known in the art.

Suitable hydrocarbonaceous feeds for the catalytic cracking process of the present invention include naphtha, hydrocarbonaceous oils boiling in the range of about 430.degree. F. (221.degree. C.) to about 1050.degree. F. (566.degree. C.), such as gas oil; heavy hydrocarbonaceous oils comprising materials boiling above 1050.degree. F. (566.degree. C.); heavy and reduced petroleum crude oil; petroleum atmospheric distillation bottoms; petroleum vacuum distillation bottoms; pitch, asphalt, bitumen, other heavy hydrocarbon residues; tar sand oils; shale oil; liquid products derived from coal liquefaction processes, and mixtures thereof.

The invention is further illustrated by the following non-limiting examples which also illustrate a preferred embodiment of the invention.

This comparative Example is directed to a catalyst composition containing zeolite and pseudoboehmite in a mesoporous matrix. 10.0 lb. (0.800 solids) (4.54 kg) of ultrastable Y zeolite and 5.7 lb. (2.6 kg) of pseudoboehmite (Davison) were mixed together dry and then dispersed in a solution of 15.0 lb. (6.81 kg) of water and 8.0 lb. (3.6 kg) of LUDOX HS-40 monodispersed silica sol (E.I. DuPont). This mixture was colloid milled and set aside. The pH of the resulting sol slurry was 5.8.

A 4 lb. (1.8 kg) dry weight batch of an acid alum/silica sol (6% Al.sub.2 O.sub.3 /94% SiO.sub.2) was prepared by reacting 30 lbs. (13.6 kg) of a 12.2% SiO.sub.2 solution (prepared by reacting 12.7 lbs. (5.72 kg) of "N" brand sodium silicate (PQ Corp) dissolved in 17.3 lbs. (7.85 kg) of water) with 9.1 lbs. (4.1 kg) of a solution containing 12.4% 98% sulfuric acid and 17.5% aluminum sulfate pentadecahydrate in a mixer such that the pH did not rise above 3.4. This batch was added immediately to the pump around. Then 5.22 lb (2.37 kg) of aluminum sulfate pentadecahydrate dissolved in 10 lb (4.54 kg) of water was added to the pump around. Finally, the zeolite/aluminum hydroxide/Ludox slurry was added to the pump around. The pH of the catalyst slurry was 3.4.

The catalyst was spray dried at the following conditions: 550.degree. F. (288.degree. C.) inlet temperature and 350.degree. F. (177.degree. C.) outlet temperature. 200 g samples of the spray dried material were continuously exchanged with 4000 g of deionized water, 4000 g of 12% ammonium sulfate solution at 74-90.degree. C., and 8000 g of deionized water. The exchanged catalysts were calcined at 550.degree. C. for 2 hours to give 131 g of Example 1.1A and 139 g of Example 1.2A. These catalysts were steamed at 760.degree. C. for 16 hours to give Examples 1.1B and 1.2B. A 10 lb. (4.5 kg) batch of the spray dried material was similarly exchanged to give Example 1.3A and this was steamed in two separate experiments at 760.degree. C. for 16 hr to give Example 1.3B and Example 1.3C.

The properties of the catalysts are summarized in Table 1.

This Example is directed to a catalyst composition containing zeolite and gibbsite in a mesoporous matrix. 10.0 lb. (0.800 solids) (4.54 kg) of ultrastable Y zeolite and 6.0 lb. (2.7 kg) of gibbsite (Alcoa) having a particle size 1.3-1.8p were mixed together dry and then dispersed in a solution of 15.0 lb. (6.81 kg) of water and 8.0 lb. (3.6 kg) of LUDOX HS-40 monodispersed silica sol (DuPont). The mixture was colloid milled and set aside. The pH of the zeolite/aluminum hydroxide/sol slurry was 5.8.

A 4 lb. (1.8 kg) dry weight batch of an acid alum/silica sol (6% Al.sub.2 O.sub.3 /94% SiO.sub.2) was prepared by reacting 30 lbs. (13.6 kg) of a 12.2% SiO.sub.2 solution (prepared by reacting 12.7 lbs. (5.77 kg) of "N" brand sodium silicate (PQ Corp) dissolved in 17.3 lbs. (7.85 kg) of water) with 9.1 lbs. (4.1 kg) of a solution containing 12.4% 98% sulfuric acid and 17.5% aluminum sulfate pentadecahydrate in a mixer such that the pH did not rise above 3.4. This batch was added immediately to the pump around. Then 5.22 lb (2.37 kg) of aluminum sulfate pentadecahydrate dissolved in 10 lb (4.54 kg) of water was added to the pump around. Finally, the zeolite/aluminum hydroxide/ Ludox slurry added to the pump around. The pH of the catalyst slurry was 3.2.

The catalyst was spray dried at the following conditions: 550.degree. F. (288.degree. C.) inlet temperature and 350.degree. F. (177.degree. C.) outlet temperature. 200 g samples of the spray dried material were continuously ion-exchanged with 4000 g deionized water, 4000 g of 12 % ammonium sulfate solution at 74-90.degree. C. and 8000 g of deionized water. The catalysts were calcined at 550.degree. C. for 2 hours to give 46 g of Example 2.1A and 50 g of Example 2.2A. These catalysts were steamed at 760.degree. C. for 16 hours to give Example 2.1B and Example 2.2B. A 10 lb. sample of the spray dried material was similarly exchanged to give Example 2.3A. This was steamed at 760.degree. C. for 16 hr to give Example 2.3B.

The properties of the above catalysts are summarized in Table 2.

This example compares the catalyst of Example 1 containing 40% zeolite and 20% pseudoboehmite in a mineral acid silica sol binder with the catalyst of Example 2 containing 40 % zeolite and 20% gibbsite in a mineral acid silica sol binder. The results are shown in FIGS. 1a and 1b in which dV is the incremental intrusion volume of mercury in a porosimeter in cc/g and dD is the change in pore diameter. D (the diameter of the pore) is given by (4.gamma. cos .theta.)/p where .GAMMA. is the surface tension of the mercury, 0 is the assumed contact angle of 140.degree. between mercury and the surface and p is the pressure. Both unsteamed catalyst (designated by the "A" at the end of the catalyst name) have little mesoporosity above 160 angstroms (FIG. 1a). While the fresh gibbsite-containing catalyst has little porosity below 150 angstroms, the pseudoboehmite-containing catalyst contains substantial porosity below 150 angstroms. On steaming at 760.degree. C. for 16 hrs. (designated by the "B" at the end of the catalyst name), the gibbsite formulation develops mesoporosity above 160 angstroms (FIG. 1b) while the pseudoboehmite formulation does not. Thus FIG. 1a shows the porosity of both the pseudoboehmite formulation (designated as Examples 1.1A-1.3A) and the gibbsite formulation (designated as Examples 2.1A-2.3A). FIG. 1b shows the porosity of these formulations after heating at 760.degree. C. As shown in FIG. 1b, the gibbsite formulation develops mesoporosity above 160 angstroms. This translates into a catalytic performance in which the gibbsite formulation makes less coke during conversion than does the pseudoboehmite formulation.

This example demonstrates that a catalyst according to the invention develops a micropore structure on steaming while the gibbsite itself does not. The results are shown in FIG. 2. FIG. 2 illustrates that gibbsite develops substantial pore structure upon calcination. The sample designated "760.degree. C. calcined gibbsite" is calcined gibbsite. Steamed Example 2 formulations display micropore structure below 100 angstroms which is consistent with less than 20% of the calcined gibbsite being present. The mesopore structure above 100 angstroms, however, reflects a feature not found in the calcined gibbsite phase or in the fresh catalyst.

This example shown that the catalyst made with 20% pseudoboehmite in Example 1 develops a micropore structure below 160 angstroms on steaming at 760.degree. C. The results are shown in FIG. 3. FIG. 3 shows that the catalyst of Example 1, upon steaming, has a pore structure which contains more pore volume than that which would be obtained with a catalyst which contained 20% 760.degree. C. calcined pseudoboehmite. In this case the pore structure which develops can be attributed entirely to the calcined pseudoboehmite present in the catalyst formulation.

This example demonstrates the the catalyst made with gibbsite produces less specific coke than a gamma alumina-containing catalyst. The evaluation test is a modified microactivity test (MAT). In this test, 2 cc of feed is injected over 5 gm of catalysts at a temperature of 482.degree. C. over a period of 80 seconds. The conversion of feed to products which boil less than 220.degree. C. is determined together with the coke and hydrogen yields. In order to allow a comparison between catalysts which produce different conversions, the coke yield and hydrogen yield are divided by a conversion function ›x/(1-x)! in which x is the conversion from the 220.degree. C. -fbp of the feed. The "normalized" coke and hydrogen yields are called the "specific coke" and the "specific hydrogen" respectively and allow comparison between catalysts of somewhat different activities. The results are shown in FIG. 4. FIG. 4 shows that the gibbsite-containing catalyst of example 2 produces less coke/kinetic conversion (specific coke) than does the catalyst made with calcined pseudoboehmite (example 1). Note that specific coke is relatively independent on conversion so that activity differences can be discounted as the source of differences in coke selectivity.